AN17
D E S I G N IN G F O R I N T E R NA TI ON AL S A F E T Y C O M P L IA NC E
Introduction
This application note is a practical guide to obtaining
certification marks for safety compliance worldwide. The
information presented here is based on a successful
submittal of the Si3034PCI-EVB evaluation board to
Nemko Compliance West. The resulting CB Test Report
and Certificate for the Si3034PCI-EVB are available
upon request.
The information provided comes primarily from the web
sites of many Certification Agencies and from a ULsponsored seminar entitled “Information Technology &
Telecommunications
Equipment:
Designing
for
Compliance to UL1950.” Also, working with Nemko
Compliance West, Inc. provided strong insight into the
Nordic Exclusions and other National Deviations.
CB Scheme
The Scheme of the IEEE for Mutual Recognition of Test
Certificates for Electrical Equipment is commonly known
as the CB Scheme. In the CB Scheme, a manufacturer
to obtains a CB Certificate for a given product. This CB
Certificate indicates that an NCB (National Certification
Body) has tested the product and found it to be in
conformance with the relevant standard. The relevant
standard for Telecommunications (IT) and Information
Technology (ITE) equipment, under the CB Scheme, is
the IEC 60950.
The process of obtaining a CB Certificate begins with
the submittal of product samples to a CB Test
Laboratory for evaluation. A CB test report is generated
based on the evaluation. If the CB test report indicates
that the product conforms to the applicable standard,
then a CB certificate is issued to formally declare the
product to be in conformance with the IEC 60950. The
CB test report is included as supporting documentation
to the CB certificate. If the product fails to conform to the
applicable standard, a special test report can be derived
from the test data, which can be useful in determining
areas that require improvement and/or redesign.
Obtaining a CB Test Report and Certificate does not
automatically mean the product has obtained the
Certification Marks required for approval in the desired
countries. The next step is to submit the CB Test Report
and certificate to the Certification Agencies for the
desired Certification Marks. The CE Marking is a
separate topic and will be discussed in a later section.
Preliminary Rev. 0.1 10/99
The procedure for submission to each Certification
Agency varies. Typically, a Certification Agency requires
a product sample and a completed Application Form in
addition to the CB Test Report and Certificate. As long
as the CB Test Report addresses all of the National
Deviations applicable to the countries to which the
Certification Marks applies, no additional testing is
required. Hence, it is important to let the CB Test
Laboratory know which countries the product targets so
that the applicable National Deviations are considered
in the generation of the CB Test Report and Certificate.
It is important to note, each Certification Agency has
differing requirements for maintaining the Certification
Mark applied to the product.
The CB Scheme, as it applies to IT and ITE equipment,
is fully accepted by the following Countries/Certification
Agencies:
Australia/SAA, Norway/NEMKO, Austria/OVE, Poland/
PCBC, Belgium/CEBEC, Canada/CSA, Singapore/PSB,
China/CCEE, Slovakia/EVPU, Czech Republic/EZU,
Slovenia/SIQ,
Denmark/DEMKO,
Finland/FIMKO,
Spain/AENOR,
France/LCIE,
Sweden/SEMKO,
Germany/TUVP-Service, Switzerland/SEV, Germany/
TUVR-Land, Germany/VDE, UK/BEAB, UK/BSI,
Hungary/MEEI, USA/USNC, India/STQC, USA/FMRC,
Ireland/NSAI, USA/ITS, Israel/SII, USA/MET, Italy/IMQ,
USA/TUVRHNA, Japan/IECEE-JP, USA/UL, Korea/
IECEE-KR, Netherlands/KEMA.
The number of CB Certificates issued is a consideration
in choosing a CB Test Laboratory. The top three
Agencies with the most number of CB Certificates
issued in 1998 for IEC 60950 (IT/ITE equipment) are
Norway/NEMKO (1395 submissions), Germany/TUVRLand (767 submissions), and USA/UL (694
submissions).
The relevant standard for Telecommunications (IT) and
Information Technology (ITE) equipment, under the CB
Scheme, is the IEC 60950. The current active version of
the IEC 60950 is the 2nd Edition, Amendments 1–4.
The IEC 60950 3rd Edition is expected to replace the
IEC 60950 2nd Edition by mid 1999.
Most of the requirements for Safety Compliance are
based on the IEC 60950. However, each country may
have requirements beyond IEC 60950. These
Copyright © 1999 by Silicon Laboratories
AN17-01
This information applies to a product under development. Its characteristics and specifications are subject to change without notice.
AN 1 7
differences, called National Deviations, are first covered
by CB Bulletins. These CB Bulletins are distributed
periodically to the NCBs and CB Testing Laboratories.
Many countries use the IEC 60950 document, plus CB
Bulletins as adequate specification for compliance.
Occasionally, there are enough differences to warrant a
separate standards document based on the IEC 60950.
An example is the UL1950 standard.
The EN 60950 is a CENELEC (Comité Européen de
Normalisation Electrotechnique) standard based on the
IEC 60950. It covers the National Deviations applicable
to the 18 member countries of the European Economic
Area, also known as the European Union (EU). The 18
member countries include the following: Austria,
Belgium, Denmark, Finland, France, Germany, Greece,
Iceland, Ireland, Italy, Liechtenstein, Luxembourg,
Netherlands, Norway, Portugal, Spain, Sweden and the
United Kingdom.
to periodic visits from a Certification Agency, because
there is no Certification Agency. Depending on the
directive and the complexity of the product, it is possible
to perform ‘self-assessment’ and declare a product to
be in conformance to the applicable directives.
However, self-assessments are more likely to be
challenged.
If a product is challenged, and it is found that the
required documentation does not support the signed
Declaration of Conformity for the specific directives that
apply to the product, then the manufacturer may be
penalized through the legal system.
Safety-Related Web Sites
There are many helpful safety related websites on the
internet. Here are just a few:
http://www.cbscheme.org
http://www.aqas.com.au/cemark1.htm
The UL1950 (3rd Edition)/CSA C22.2 No. 950-95 (Third
Edition) is a bi-national standard based on the IEC
60950. UL1950 applies to the United States and
Canada.
http://www.aqas.com.au/eec.htm
CE Marking
http://www.iec.ch
The CE Mark is a “passport” which permits a product’s
entry into the 18 member countries of the European
Economic Area. It is important to note that the CE
Marking is not a Certification Mark issued by a
Certification Agency under the CB Scheme.
http://www.us.tuv.com/library/resources/index.html
The CE Marking, along with a Declaration of Conformity
and Technical File, is a manufacturer’s declaration that
the product complies with the directives applicable to
the product. The directives that apply to IT and ITE
equipment are as follows: Low Voltage Directive (LVD),
Electromagnetic Emissions Directive (EMC) and the
Machinery Directive (with moving parts).
http://global.ihs.com/
Typically, a product is submitted to a Notified Body.
Notified Bodies are authorized by European countries to
serve as independent test labs and perform the steps
called out by product directives. They must have the
necessary qualifications to meet the testing
requirements set forth in the directives. Notified Bodies
may be private sector organizations or government
agencies. Manufacturers may choose a Notified Body in
any member state of the European Union.
The Low Voltage Directive (CE Marking Directive 73/23/
EEC as amended by 93/68/EEC) applies to product
safety related to electrically operated devices. The EN
60950 is used to show conformity to Annex I (Essential
Requirements) of the LVD.
With the CE Marking, the manufacturer is not subjected
2
http://www.nemko.no
http://www.ul.com
http://www.csa-international.org/links/index.html
http://www.ce-mark.com/cefree.html
http://www.eurunion.org/infores/standard.htm
Steps to Safety Compliance
A Professional Testing Agency is a privately or publicly
owned consultation company that can act in the
capacity of a CB Test Laboratory, an agent to an NCB,
an NCB, a CE Notified Body, an agent to a Certification
Agency, a safety consultant, as well as in other
capacities. The Professional Testing Agency is in the
business of providing services for compliance testing.
When looking for a Professional Testing Agency, a good
place to start is the list of Certification Agencies. Many
of these Certification Agencies are also CB Test
Laboratories, NCBs, and CE Notified Bodies.
The general steps in getting CE Marking, CB Test
Report and Certificate, and other Certification Marks are
as follows:
1. Product Development Engineering should assess
the product for safety-related issues. If UL1950
compliance is required, there are system-level
construction issues that need to be considered at
this time.
Preliminary Rev. 0.1
AN17
2. Assign a Compliance Test Engineer who is familiar
with your product to be tested. The Compliance Test
Engineer will work with the Professional Testing
Agencies. There may be more than one agency
involved, depending on the services provided by the
Testing Agency.
3. Select a Professional Testing Agency. The following
is a checklist for choosing a testing agency:
• Has Test Setups required for IEC 60950 electrical
testing
• Receives CB Bulletins on a regular basis
• Has Knowledge of National Deviations pertaining
to the countries you want to enter. At the very
least, must be familiar with EN 60950
• Has UL1950 3rd Edition Overvoltage Test Setup if
North America is included in the target countries
• Has a good understanding of how to achieve
UL1950 3rd Edition compliance
• Can act as an agent on your behalf, for submission
of CB Test Report and Certificate to obtain
Certification Marks required
• Has experience writing CB Test Reports and
Certificates for ITE/TE appliances.
• Has experience preparing Declaration of
Conformance and Test Files for CE Marking
4. Submit samples of your product to the chosen
Professional Testing Agency to do the actual
electrical test and measurement of creepage and
clearance. If UL 3rd Edition Overvoltage Testing is
involved, it is important to plan ahead as to the
number of overvoltage tests that will be done. The
overvoltage tests may render the product inoperable
after the tests.
other CE directives are met (e.g., EMC Directive),
and all documents are in order, then the product may
be shipped with the CE Marking.
9. Work with the Professional Testing Agency to submit
a sample of the product, CB Test Report and
Certificate, and Signed Application Forms (supplied
by Certification Agency) to other Certification
Agencies for obtaining their Certification Marks.
Design Considerations for Product Safety
This section discusses the treatment of the principles of
safety as it relates to the modem circuit. There are other
considerations of safety such as power supply issues,
and fire enclosures. Safety issues outside the scope of
the modem sub-system are best handled through
consultation with the Professional Testing Agency.
2.5-mm Isolation Barrier
In all modem designs, there is a portion of the modem
which is isolated from the local ground. These isolated
components are part of an IEC 60950 classification type
called the TNV-3 (Telecommunications Network Voltage)
Circuit. TNV-3 circuits are subject to ringing voltages
and lightning surges. TNV-3 circuits are not
considered safe to touch by the user.
Circuits powered by low-voltage DC supplies in which
no hazardous voltages are generated are called SELV
(Safety Extra Low Voltage) circuits. SELV circuits are
safe to touch by the user and include the local ground.
All components from Tip/Ring to the Si3014 (Line Side)
device are considered to be in the TNV-3 Circuit area.
The Si3021 (DSP-side) is considered to be on the SELV
Circuit area. Figure 1 shows the SELV and TNV areas.
5. The Professional Testing Agency writes the CB Test
Report when the product meets the requirements for
safety.
6. The Professional Testing Agency submits the CB
Test Report to the NCB to obtain the CB Certificate.
Typically, the first Certification Mark is obtained from
the NCB issuing the CB Certificate.
7. The CB Test Report can be used to show
conformance to the LVD Directive. The Professional
Testing Agency compiles the CE Test File, which
contains a copy of the CB Test Report.
8. The Professional Testing Agency compiles the
Declaration of Conformity for the LVD Directive. An
officer of the company will sign the Declaration of
Conformity for the LVD Directive. Assuming that the
Preliminary Rev. 0.1
3
AN 1 7
TNV-3
Isolation Barrier
Diode
Bridge
Discretes
Si3014
Si3021
C24
Figure 1. Si3034 Modem Design Overview
The boundary between the TNV-3 and SELV Circuit
area is frequently called the Isolation Barrier. In an
Si3034 design, the Isolation Barrier is between the
Si3021 and Si3014. The IEC 60950 term for a
separation between the SELV and TNV-3 is the
application of “Insulation” between these two circuit
types.
There are many classifications for insulation between
the TNV-3 and SELV. The most common requirement is
that of “Basic Insulation”. “Basic Insulation” defines the
required distance across the Isolation barrier.
accumulation problem. Hence, creepage distance is
always larger than clearance. In the case of TNV-3 to
SELV separation, the required minimum clearance is
2.0 mm. Since both clearance and creepage distance
requirements need to be met, the larger value of
creepage distance is used.
Figure 2 shows an example of creepage distance and
clearance.
The separation between the TNV-3 and the SELV
portion of the circuit is also a function of the normal
working voltage expected between the TNV-3 and the
SELV, as well as the expected conductive dust
conditions that may accumulate on the Isolation Barrier.
For an international modem, under the worse case
conditions of ringing voltages and conductive dust
particle pollution, the required minimum distance is
2.5 mm. This 2.5 mm distance is to be applied from any
PCB trace between the TNV-3 area to SELV circuit and
from any PCB trace between TNV-3 and local ground.
This distance is measured by skimming the surface of
the PCB and is called “creepage” in IEC 60950
terminology. Creepage distance is applied to prevent
electrical arcing across conductors as a result of dust
particle accumulation over a surface. Dust particle
accumulation, over time, can degrade the isolation
between conductors. Distance through air, called
“clearance”, does not have this dust particle
4
Preliminary Rev. 0.1
AN17
Conductor
Conductor
Clearance
Creepage Distance
Figure 2. Creepage vs. Clearance
TNV Cover
The IEC 60950 states that TNV-3 circuits must not be
accessible to a casual user. The casual user is referred
to as the “operator” in the IEC 60950. As mentioned
previously, TNV-3 circuits are not considered touchable
by operators. In the example of a PCI Modem inside a
personal computer (PC), some countries consider
operating a PC without the enclosure as a normal
operating environment. As such, these countries will
require a cover around the TNV-3 area.
The TNV cover is intended to prevent an operator from
touching live TNV-3 circuits when the PC is used
without its primary enclosure. If the cover is made of
plastic, then there are no distance requirements
between any exposed TNV-3 conductor and the plastic
cover.
If the cover is made of a conductive material, then the
minimum distance of 2.5 mm “creepage” applies to the
base of the metallic TNV cover and any TNV-3 trace.
This 2.5 mm creepage distance is measured along the
surface of the PCB.
As indicated previously, the air-distance called
“clearance” is required to be 2.0 mm, not 2.5 mm.
Hence, the height of a metallic TNV cover is determined
by the tallest conductive component on the TNV-3
Circuit area plus 2.0 mm.
To build an international modem, it is best to make
provisions for the TNV Cover. But for cost purposes,
ship the TNV cover only to countries that require them.
The Professional Testing Agency should be
knowledgeable as to which countries require a TNV
cover.
For instance, North America does not require the TNV
cover as long as the documentation warns the user or
operator to unplug the phone cord while installing the
modem.
Isolation Barrier Capacitors
In many designs, including the Si3034PCI-EVB,
capacitors are used to bridge the Isolation Barrier. For
the Si3034PCI-EVB, these are C1, C2, C4, C24, and
C25. The IEC 60950 requires that this Isolation Barrier
be suitable for “Basic Insulation”. Figure 1 shows an
overview of how the isolation capacitors are used.
Essentially, a capacitor that is able to reach across the
2.5 mm Isolation Barrier is considered to be “Basic
Insulation.” Conceptually, a capacitor is constructed so
that two metal plates are separated by a suitable
dielectric material. The dielectric acts as an insulator
between the two metal plates. As long as the capacitor
is able to withstand the “Dielectric Strength Tests,” it
qualifies as “Basic Insulation.” In the process of having
your product tested, the capacitors will be subjected to
these electrical tests. High voltage capacitors (2000 V)
and Y2-class capacitors are able to withstand these
tests.
When submitting the product for testing, it is
recommended that alternate sources for the Isolation
Barrier capacitors be identified and tested. Not doing so
may result in the need for re-testing and re-qualification
of the system if a capacitor supplier is unable to meet
delivery.
Another item to consider are the footnotes in the IEC
60950 that require the isolation be upgraded to
“Supplementary Insulation” for Nordic Countries. These
footnotes are referred to as the “Nordic Exclusions”.
The Nordic Countries are Norway, Sweden, Finland and
Denmark.
Preliminary Rev. 0.1
5
AN 1 7
“Supplementary Insulation” has all the requirements of
“Basic Insulation.” In addition, the thickness of the
dielectric material between the poles of the capacitor is
governed by a “Minimum Distance Through Insulation”
of 0.4 mm. Figure 3 illustrates “distance through
insulation.”
Insulator or Dielectric
Conductor
Conductor
The UL1950 3rd Edition overvoltage tests are required
only for the United States and Canada. After April 1,
2000, new product submissions need to be evaluated
against the UL1950 3rd Edition. Existing products that
were evaluated using other standards do not need to be
reevaluated until April 1, 2005. These overvoltage tests
is due to the co-location of phone lines and power lines
on the same utility pole network. In the event of a
failure, such as a tree falling over utility lines, it is
possible that the lines could cross, sending hazardous
voltages across the phone line. This is not the case in
Europe because the phone lines and power lines are on
separate utility poles, or the utility lines are buried.
The UL1950 3rd Edition overvoltage tests are defined in
Subclause 6.6 and Annex NAC. While designing your
product, it is important to obtain copies of the relevant
standards for your reference.
“Distance through Insulation”
Figure 3. Distance through Insulation
Given this design constraint, the capacitor becomes
physically larger. The IEC384-14 is the standard that
governs the construction and the certification of a
capacitor suitable for applications that can be used to
bridge “Supplementary Insulation.” The capacitor suited
for “Supplementary Insulation” is defined as a Y2
capacitor. Besides the minimum distance through
insulation, a further design constraint of the Y2
capacitor is a 4-mm creepage distance between
opposite poles of the capacitor. This design constraint
requires a surface mount capacitor to be of size EIA
22xx or larger.
For applications that do not have significant height
constraints, it is recommended that ceramic disk
packaging be used in lieu of a surface mount. The
ceramic disk packaging requires two holes separated by
7.5 mm. There are many manufacturers who build Y2
capacitors that use the same 7.5 mm footprint. Among
them are Panasonic, Murata, and Samsung. An
inexpensive solution is to use all through-hole.
A surface mount Y2 capacitor of size EIA 2220 is
manufactured by Murata. Another option is to create
dual footprint pad sites that can host either an EIA 2220
or an EIA 1808 capacitor. The resulting pad sites are “T”
shaped. This way Y2 capacitors are used only for the
Nordic countries and standard EIA 1808 capacitors for
the rest of the world.
6
UL1950 3rd Edition Overview
Assuming that the system is subjected to overvoltage
tests, voltage is applied to the system as a “differential
mode” or a “common mode.” For the “differential” mode,
also known as “metallic,” voltage is applied between Tip
and Ring, while other accessible conductors are
grounded. For the “common mode” tests, Tip and Ring
are tied together, and all other conductors are tied to
ground. A voltage is then applied to both Tip and Ring.
Another item of consideration is whether or not the
modem is placed “on-hook” or “off-hook.” For the
Si3034-based design, the loop current is actively
controlled through Q3. However this can be controlled
only when the system is powered on. When the system
is powered off, the system is always on-hook.
During safety testing, the system is not powered on. To
simulate an off-hook condition, the emitter and collector
of Q3 are shorted together. This shorting of the collector
and emitter of Q3 represents an artificial worse case
condition that will not happen in a real system. If the
system were powered on as it needs to be to go off
hook, excessive loop currents would be detected and
the system would be placed in the on-hook position
before any damage could be inflicted on the modem.
Even with the artificial condition of a shorted Q3, the
submitted Si3034-based design passed. Figure 4
shows the devices most affected by the off-hook
condition.
Preliminary Rev. 0.1
AN17
2.2 A
U2
D1
Q1
FB1
Tip
+
Si3014
200 V, RV1 not clamped, fuse
not blown, Q3 VCE shorted
Q2
–
FB2
Z1
Ring
D2
2.2 A
Q3 CE Short
Figure 4. Devices Most Affected by Overvoltages and Overcurrents, Off-Hook Case
In all of the overvoltage tests, two layers of cheesecloth
are tightly wrapped around the system, or subassembly. To pass the tests, the cheesecloth must not
ignite. The key concept to remember is that the
manufacturer has the choice of where the cheesecloth
is applied by defining the assembly, or sub-assembly.
For instance, for a PCI Modem, it is possible to test the
PCI Modem sub-assembly separate from the PC. If the
PCI Modem sub-assembly is tested outside the PC
system, the cheesecloth is wrapped around the PCI
modem sub-assembly. The PC manufacturer may
choose to test the entire PC system with the modem
already installed. It is up to the manufacturer to define
the boundary of the system. The goal is to show
compliance to the UL1950 standard. It will be shown
later how overvoltage testing can be minimized.
To pass the overvoltage tests, the cheesecloth around
the system or sub-assembly should not ignite or char.
After each of the overvoltage tests, the system is
subject to a dielectric strength test to ensure that the
Isolation Barrier is intact. UL1950 3rd Edition defines
five overvoltage tests.
Test 1 and Test 5 are not discussed. It will be shown
later that it is simple to find a way of bypassing Test 1. It
will also be shown later that Test 5 is usually
unnecessary.
Test 2 subjects the system to 600 V at 7 A for
5 seconds. It is important to note that this is the only test
conducted that has a maximum current of 7 A
(assuming Test 1 is omitted).
Test 3 subjects the system to 600 V at 2.2 A for
30 minutes. An additional test, Test 3a, is done only if
an open circuit results from Test 3. For Test 3a, the fuse
is shorted and the system is subjected to 600 V at 135% of
the fuse rating for 30 minutes.
Test 4 subjects the system to a voltage just below the
tripping voltage of a voltage protection device
(Sidactor). For example, 200 V at 2.2 A for 30 minutes.
When overvoltage tests are performed, a passing mark
means the cheesecloth did not char or catch fire, the
wiring simulator did not open (test 1 and 5 only), and the
dielectric strength test (conducted after each
overvoltage test) shows the isolation barrier is not
damaged.
Minimizing UL 3rd Edition Overvoltage Tests
This application note presents Table 18b of the UL1950
specification in a slightly different way. This method of
presentation is designed to highlight the effect of system
elements on the required overvoltage tests. Table 1
shows a simplified matrix to determine if Overvoltage
Tests 1 and 5 are required. Test 1 and the requirement
of 26 AWG wire are directly linked. Test 5 and the
requirement of passing Subclause 6.3.3 of the IEC
60950 is directly linked. Subclause 6.3.3 defines the
requirements for separation of the telecommunications
network from earth ground.
It is important to remember that the goal of the
overvoltage tests is to reduce the risk of fire. As such,
there are non-electrical system elements such as fire
enclosures and spacings to consider.
In most cases, the system will not have to be subjected
to Test 1. To remove the requirement for Test 1, there
must be a warning in the documentation or on the
product. An example is shown below:
CAUTION -- To reduce the risk of fire, use only No. 26 AWG
or larger telecommunication line cord.
Preliminary Rev. 0.1
7
AN 1 7
Another method of skipping Test 1 is to supply a phone
cord (AWG 26) with the product, as well as sufficient
instructions to indicate that the product must be used
with the enclosed phone cord, or an equivalent phone
cord, to reduce the risk of fire.
The manufacturer only needs to guarantee that the wire
from the modem to the phone jack on the wall uses 26
AWG cord. The wire within the building is not included in
the safety assessment because it is the builder’s
responsibility to use 26 AWG phone wires throughout
the building.
Table 2. System Element vs. Overvoltage Tests
Overvoltage
System Elements
Tests
Fire
Fuse Enclosure Spacing
Test 2
Test 3,
3a, 4
No
No
Don’t
Care
Yes
No
Don’t
Care
No
Yes
No
OverVoltage
No
Yes
Yes
Skip
Skip
System Element
Tests
Yes
Yes
Skip
Skip
26 AWG
Test 1
Don’t
Care
No
Required
Yes
Skip
Table 1. System Element vs. Overvoltage Tests
Tests
Pass 6.3.3
Test 5
No
Required
Yes
Skip
Skip
Required
Required Required
If a fuse is used, the fuse must be 100 A2-s limiting and
have a 1.3 A maximum steady state current. Typically, a
fuse or PTC manufacturer will state in their literature
that the product is compliant with UL1950/UL1459
power cross tests. Surface mount fuses are available. In
choosing a fuse or PTC, consult with a UL engineer to
ensure that it is suitable for this application.
OverVoltage
System Element
Required Required
Test 5 can be skipped if the product complies with the
testing of Subclause 6.3.3 of the IEC 60950. In the
process of performing the tests for compliance to the
IEC 60950, this is already done. If for some reason it
has not been done, then the system is subjected to a
125 V test.
Table 2 shows the remaining overvoltage test (Tests 2,
3, 3A, and 4) required for different system configuration.
A fire enclosure is used to prevent the spread of a fire
originating from the equipment. The precise
characteristics that make a fire enclosure are beyond
the scope of this paper. The important thing to
remember is that the system element of a fire enclosure
is key to eliminating the overvoltage tests. A good
example for a fire enclosure is a PC Chassis.
The system element ‘spacing’ on Table 2 refers to an air
distance of 25 mm between the TNV-3 circuit and
materials of flammability V-2 or worse. No spacing is
required if the TNV-3 circuit is next to materials of class
V-1 or better. In addition, if the TNV-3 circuit is adjacent
to an opening on the fire enclosure, then there are
restrictions to the size of the openings on the enclosure.
If the material adjacent to the TNV-3 circuit is unknown
or unspecified, it is assumed to be of flammability V-2 or
worse.
Table 2 illustrates that it is possible to skip all of the
overvoltage tests under certain conditions. It also shows
that inclusion of a fuse in the system has limited value.
A fuse without a fire enclosure makes it possible to skip
Test 2. But, since Tests 3 and Tests 4 are required
anyway, it may make sense to simply not have a fuse
and subject the system to Tests 2, 3 and 4. The only
value in skipping Test 2 is that Test 2 is a 7 A test, while
the other tests are 2.2 A tests.
8
Preliminary Rev. 0.1
AN17
A fuse in a system with a fire enclosure, provides a
slight advantage by eliminating the requirement of
spacing between the TNV-3 circuit and adjacent
materials.
The Si3034PCI-EVB has an official CB Test Report and
Certificate. The system was subjected to all of the
required tests for the IEC 60950 and all of the known
National Deviations, including those of North America.
A PCI Modem being tested by a PC manufacturer is a
good example. A PCI Modem is mounted into a PCI
slot. Perhaps the card is facing the rear of a Video Card.
As long as there are no tall components mounted on the
rear of the Video Card, then the minimum spacing of
25 mm is met simply because the PCI slots are
separated by slightly more than 25 mm by design. The
PC chassis is considered a fire enclosure. Assuming
that the user instructions include directions that indicate
the phone cord must be of 26 AWG or better, no
overvoltage tests are needed. If the phone cord of 26
AWG is supplied, the system will not require
overvoltage tests.
The Isolation capacitors easily passed electrical testing.
The tests included the required impulse testing, followed
by dielectric strength tests, and finally insulation
resistance measurements. The Si3034PCI- EVB was
submitted with many different capacitor configurations
and with multiple sources (Venkel and Novacap). All
configurations passed.
There are many options available for compliance to the
UL1950. As mentioned earlier, construction of the
system has a large effect on the number and severity of
the overvoltage tests required.
Consider the scenario in which this system is subjected
to an actual power cross.
No assumptions were made as to whether or not a fire
enclosure existed and the Si3034PCI-EVB was tested
at the sub-assembly level with the cheesecloth wrapped
tightly around the sub-assembly.
The cord from the wall to the PC will not overheat and
present a fire hazard because the phone cord is 26
AWG or better.
The Si3034PCI-EVB passes all overcurrent and overvoltage tests for UL1950 3rd Edition compliance with
minor circuit modifications.
The worse case scenario is that the PCI modem ignites.
If the PCI modem ignites, it is separated from the video
card by 25 mm. At the very worst, it will expel carbon
debris to the back of the video card. The PC chassis
being a fire enclosure, provides another level of
protection.
Figure 5 shows the designs that can pass the UL1950
overvoltage tests, as well as electromagnetic
emissions. The top schematic of Figure 5 shows the
configuration in which the ferrite beads (FB1, FB2) are
on the unprotected side of the sidactor (RV1). For this
configuration, the current rating of the ferrite beads
needs to be 6 A. However, the higher current ferrite
beads are less effective in reducing electromagnetic
emissions, thus requiring the capacitors (C24, C25) to
pass electromagnetic emissions.
Consequently, the sample system above passes the
requirements of UL1950 without subjecting the system
to overvoltage tests or adding a fuse. The system
passes the UL1950 3rd Edition simply by
documentation and fire enclosure construction.
In the case of a PCMCIA modem, the modem is
assumed to be enclosed in a small metal enclosure. If
the PCMCIA card enclosure is designed with V-1
material around the TNV-3 circuit, then no overvoltage
testing or fuse is required.
Now consider a laptop with an integrated modem on the
motherboard. Depending on the enclosure material
used, the laptop may not be considered a fire enclosure.
In this case, a metal enclosure around the TNV-3 circuit
may need to be designed so that overvoltage testing
can be bypassed. Another option is to omit any fire
enclosure and subject the laptop to overvoltage testing.
Submission of the Si3034PCI-EVB
In picking a CB Test Laboratory, the manufacturer
needs to take into consideration existing relationships,
the cost of the tests, certification programs available,
location of the test facility, and many other factors.
The bottom schematic of Figure 5 shows the
configuration in which the ferrite beads (FB1, FB2) are
on the protected side of the sidactor (RV1). For this
design, the ferrite beads can be rated at 200 mA. It is
possible to use higher impedance ferrite beads without
requiring C24 and C25 to aid in passing
electromagnetic emissions.
Conclusion
It is important to remember that compliance to UL1950
does not always require overvoltage tests. It is best to
plan ahead and know which overvoltage tests will apply
to your system. System-level elements in the
construction need to be considered during the design
stages. Consult with your Professional Testing Agency
during the design of the product to determine which
tests apply to your system.
Preliminary Rev. 0.1
9
AN 1 7
C24
75 @ 100 MHz, 6 A
1.25 A
FB1
Tip
75 @ 100 MHz, 6 A
RV1
FB2
ing
Note: In this configuration, C24 and C25 are
used for emissions testing.
1000 @ 100 MHz, 200 mA
1.25 A
FB1
Tip
1000 @ 100 MHz, 200 mA
RV1
FB2
Ring
Figure 5. Circuits that Pass all UL1950 Overvoltage Tests
10
Preliminary Rev. 0.1
AN17
NOTES:
Preliminary
11
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